Enable loading of external Firebird dialect, if available

Fixes: #5278
Change-Id: I1660abb11c02656fbf388f2f9c4257075111be58
(cherry picked from commit 8c5aa7d3e0)

doc/build/changelog/unreleased_13/5278.rst

@@ -0,0 +1,7 @@
+.. change::
+    :tags: change, firebird
+    :tickets: 5278
+
+    Adjusted dialect loading for ``firebird://`` URIs so the external
+    sqlalchemy-firebird dialect will be used if it has been installed,
+    otherwise fall back to the (now deprecated) internal Firebird dialect.

lib/sqlalchemy/dialects/__init__.py

@@ -42,7 +42,14 @@ def _auto_fn(name):
         )
         dialect = translated
     try:
-        module = __import__("sqlalchemy.dialects.%s" % (dialect,)).dialects
+        if dialect == "firebird":
+            try:
+                module = __import__("sqlalchemy_firebird")
+                dialect = "dialect"
+            except:
+                module = __import__("sqlalchemy.dialects.firebird").dialects
+        else:
+            module = __import__("sqlalchemy.dialects.%s" % (dialect,)).dialects
     except ImportError:
         return None